1. Field of the Invention
The present invention relates to a circuit having an input amplifier and a second amplifier that provides the circuit with a unity gain crossover frequency that is higher than a unity gain crossover frequency of the input amplifier. The circuit further includes a control input being coupled to a control input of the input amplifier and also includes a first current connection and a second current connection.
2. Description of the Background Art
A unity gain crossover frequency of an amplifier element is understood by one skilled in the art to mean the frequency at which its frequency-dependent amplification, e.g., a quotient of an output current value and input current value, drops to a value of one. A circuit of this nature, also known as a unity gain crossover frequency multiplier or unity gain crossover frequency doubler, is known per se. In this context, the term multiplier is to be interpreted such that increases in the unity gain crossover frequency by non-integer factors <1 is also considered to be multiplication.
As an example of such a circuit that is known per se, reference is made to a cascode such as is described in Tietze-Schenk, Halbleiter-Schaltungstechnik, 9th edition, Springer Verlag, p. 492. This cascode has an input amplifier in the form of a bipolar transistor connected in a common-emitter circuit, the collector of which is coupled to the emitter of a second transistor that is connected in a common-base circuit. The coupling with the second transistor in a common-base circuit eliminates the Miller effect.
However, unity gain crossover frequency multipliers can also be realized using other circuit topologies having normal bipolar transistors, SiGe heterojunction bipolar transistors (SiGe HBT), HBTs made of III/V semiconductors, field-effect transistors and/or operational amplifiers. However, this listing of transistor types makes no claim to completeness and thus is not to be construed as limiting.
In the most general sense, the primary advantage of unity gain crossover frequency multipliers is that they can replace amplifier elements, and in particular individual transistors, as separate elements in a high-frequency circuit design.
Despite ongoing development of high-frequency technologies and their diverse applications in the millimeter and sub-millimeter wavelength regions (frequencies in the gigahertz to terahertz range), there are many microwave and optoelectronic applications which demand further improvements in high-frequency characteristics and other electrical characteristics, especially increases in unity gain crossover frequency, improvements in the shape of the gain characteristic, increases in breakdown voltage and power flow through amplifier elements, etc.